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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109945] Escape analysis hates copy elision:
 different result with -O1 vs -O2
Date: Tue, 23 May 2023 17:46:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109945

--- Comment #7 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
Richard Biener wrote:
> Are we using the wrong check or is escaping 'this'
> for these kind of classes invoking undefined behavior?

Wow, this got a lot of traffic quickly!  Sounds like you (Richard, Andrew) =
are
on top of the issue here, where a constructor is involved; but once you sta=
rt
talking about heuristics and checks, I think it would be productive for you=
 two
to make sure you're both on the same page with *this* example:

// https://godbolt.org/z/Ea43Y65z4
struct Widget {
    int i =3D 1;
    int a[4];
};
Widget *global =3D nullptr;
Widget make2() { Widget w; global =3D &w; return w; }
void g() { global->i =3D 42; }
int main() {
  Widget w =3D make2();
  int i =3D w.i;
  g();
  return (i =3D=3D w.i);
    // Does this need to be reloaded and
    // compared? or is it obviously true?=20=20
}

In this case, Widget has no constructors, and I think it would be perfectly
defensible for the compiler to say that "global =3D &w" is not a valid way =
to
escape the address of that prvalue result (even though copy elision will ma=
ke
the w in make() and the w in main() the same object). But do both of *you* =
have
the same answer to that paradox? If you don't, then you might also not agree
about what the appropriate heuristic should be in the original case and mig=
ht
end up talking past each other.=